1. Field of the Invention
The present invention relates to a transceiving filter for selecting a transmission signal and/or a reception signal and allowing these signals to pass. The present invention further relates to a communication device including this transceiving filter.
2. Description of the Related Art
In the past, a base station of a mobile communication system has a transceiving filter including an amplifier circuit for amplifying a reception signal between an antenna at the top of a tower and a transceiving circuit.
For example, International Application Published under the Patent Cooperation Treaty No. 02/31997 (hereinafter referred to as a patent document 1) discloses a transceiving filter including first and second transmission paths between a first port to which an antenna can be connected and a second port to which a transceiving circuit can be connected.
FIG. 6 illustrates the configuration of the above-described transceiving filter. As shown in this drawing, an antenna ANT, a transceiving circuit 10, a first transmission path P1 for allowing a reception signal to pass, and a second transmission path P2 for allowing a transmission signal to pass. Reception filters Rx1 and Rx2 with a band pass characteristic for allowing the reception signal to pass and an amplifier circuit LNA are provided on the first transmission path P1. A transmission filter Tx1 with a band attenuation characteristic for attenuating only the reception signal is provided on the second transmission path P2.
According to the above-described configuration, a reception signal input from the antenna ANT is amplified in the first transmission path P1 and transmitted to the transceiving circuit 10. A transmission signal transmitted from the transceiving circuit 10 is transmitted to the antenna ANT via the second transmission path P2.
Further, International Application Published under the Patent Cooperation Treaty No. 01/35684 (hereinafter referred to as a patent document 2) discloses a transceiving filter having a first directional circuit at a first junction of a first transmission path and a second transmission path. This first directional circuit is provided for transmitting a reception signal from a first port toward a second port via the first transmission path. The transceiving filter further has a second directional circuit at a second junction of the first transmission path and the second transmission path. The second directional circuit is provided for transmitting a transmission signal from the second port toward the first port via the second transmission path.
The first directional circuit includes at least one first circulator for transmitting the reception signal input from the first port to the amplifier circuit and transmitting the transmission signal from the second transmission path to the antenna, and the second directional circuit includes at least one second circulator for transmitting the reception signal amplified by an amplifier circuit to the second port and transmitting the transmission signal input from the second port to the second transmission path.
FIG. 5 illustrates an example configuration of the patent document 2. In this example, two reception filters Rx1 and Rx2, and an amplifier circuit LNA are provided on a first transmission path P1. A circulator 15 having first ports (1), (2), and (3) is provided at a first junction of the first transmission path P1 and a second transmission path P2. Further, a circulator 16 having second ports (1), (2), and (3) is provided at a second junction of the first transmission path P1 and the second transmission path P2. A direction from the first port (1) to the first port (2) and a direction from the first port (2) to the first port (3) are determined to be a forward direction of the circulator 15. A direction from the second port (1) to the second port (2) and a direction from the second port (2) to the second port (3) are determined to be a forward direction of the circulator 16. Subsequently, a reception signal input from a port #1 is input to the first port (2) of the circulator 15 and transmitted from the first port (3) to the first transmission path P1. The reception signal is amplified by the LNA, input to the second port (3) of the circulator 16, and output from the second port (1) to a port #2.
A transmission signal input from a transceiving circuit 10 to the port #2 is transmitted from the second port (1) to the second port (2) and further transmitted to the first port (1) of the circulator 15 via the second transmission path P2. Then, the transmission signal is transmitted from the first port (1) to the first port (2) and output to an antenna ANT via the port #1.
However, in the case of FIG. 6, the first and second transmission paths P1 and P2 are provided and the LNA is provided on the first transmission path P1. These two transmission paths often form a feedback loop. The reception filter Tx1 is provided for allowing a signal in a transmit frequency band to pass and attenuating a signal in a receive frequency band. If attenuation near this receive frequency band is inadequate, an output signal from the LNA is positively fed back to the LNA via the reception filter Rx2, the transmission filter Tx1, and the reception filter Rx1 in this order, whereby this output signal oscillates. Therefore, a large attenuation near the receive frequency band of the transmission filter Tx1 should be achieved so that the gain of the feedback loop bercomes 1 dB or less, whereby the signal oscillation reduces. In this case, it becomes necessary to increase the width of an attenuation band of the receive frequency band of the transmission filter Tx1 and/or decrease the width of a transmit frequency band of each of the reception filters Rx1 and Rx2. Consequently, the insertion loss (IL) of the transmit frequency band and/or that of the receive frequency band, and the noise figures (NF) of this transceiving filter increase. Further, the group-delay (GD) characteristic of this transceiving filter deteriorates.
Where the transceiving filter with the circulators 16 and 15 shown in FIG. 5 is used and two signals whose frequencies are different from each other are input, as transmission signals, from the transceiving circuit 10 to the circulators 16 and 15, intermodulation (IM) distortion occurs in the circulators 16 and 15, whereby IM-distortion signals are output from the second ports (2) and (3) of the circulator 16. For example, where high-power circulators are used and transmission power of 45 dBm is input to the transceiving filter, the tertiary IM-distortion level in the receive frequency band becomes high so as to be about −80 dBm. This significant IM distortion occurs due to the characteristic of ferrite used for making the transceiving filter nonreciprocal.
If a tertiary IM-distortion signal in the receive frequency band is output from the first port (2) of the circulator 15, this signal is transmitted, as a disturbing wave, from the antenna to a nearby base station. Subsequently, the reception sensitivity of the base station deteriorates. If the tertiary IM-distortion signal is output from the first port (3) of the circulator 15, it is amplified by the LNA, whereby the reception sensitivity of a base station of this transceiving filter deteriorates.
The allowable value of a tertiary IM-distortion signal generated by transmission paths of currently used systems is about −100 to −120 dBm. However, it is difficult for the transceiving filter with the above-described configuration including the circulators to maintain the allowable level.